This invention relates to application methods and apparatuses for applying uniformly thick coatings on flat substrates.
In the photolithographic process, a polymeric photo resist layer is formed on a thin film to be etched and then exposed to actinic radiation through a photo mask, e.g., by contact printing. Actinic radiation renders one portion of the photo resist relatively more soluble, and the other portion relatively less soluble. The more soluble portion of the photo resist is removed, e.g., by solubilization with a suitable solvent, uncovering portions of the thin film. The uncovered portions of the thin film are then removed by etching, leaving behind a facsimile or reverse facsimile of the photo mask pattern. However, variations in the thickness of the applied polymeric photo resist layer generate imperfections in the photo mask pattern. One way to ensure a uniformly thick polymeric photo resist layer is to apply highly viscous resist.
Techniques which have been developed for formation of the film of the photometric photo resist include: meniscus coating (see U.S. Pat. No. 5,270,079 incorporated herein by reference), slot coating (see U.S. Pat. No. 4,696,885 incorporated herein by reference), and patch coating (see U.S. Pat. No. 4,938,994 incorporated herein by reference). However, these methods have many disadvantages; for example, there is excess waste of the photo resist, only substrates of limited size may be accommodated, they produce a film coating of non-uniform thickness, and they produce an edge bead build up at the end of the coating. In particular, the meniscus coating method allows too much solvent evaporation and patch coating is a complicated, unproven technique.
Capillary coating is a superior method that applies a more uniformly thick layer of photo resist to substrates, does not produce an edge bead build up, can handle larger substrate sizes, and does not allow solvent to evaporate. Referring to FIG. 1, a capillary apparatus is shown wherein the substrate (1) is held by a vacuum chuck (2). The vacuum chuck (2) is positioned above a coat head (3) of photo resist. The coat head (3) of photo resist is in fluid communication with a photo resist reservoir (4). The photo resist travels from the reservoir (4) up through the coat head (3) to an orifice at the top of the coat head where it forms a meniscus. The vacuum chuck (2) is then made to bring the substrate (1) into contact with the meniscus. The vacuum chuck (2) then moves the substrate (1) laterally, relative to the coat head (3). As the vacuum chuck (2) is moved horizontally relative to the coat head (3), a layer (5) of photo resist is coated onto the substrate (1). This method provides a uniformly thick layer because the flow of resist onto the substrate is adequately controlled by the capillary action. However, capillary action is only effective for fluids having a viscosity of less than thirty centipoises, because the capillary action is not strong enough to pull highly viscous liquids up through the coat head (3).
Therefore, there is a need for a capillary type method of applying fluids having viscosities of greater than thirty centipoises.
The present invention introduces a slight pressure differential between the photo resist at the bottom of the coat head and at the meniscus to assist the capillary action in moving viscous liquid up the coat head. This allows capillary systems to apply fluids having viscosities of greater than thirty centipoises. This method may be used to apply any fluid, within the above parameters, for any purpose, as understood by those skilled in the art, besides the photolithographic process identified above.
According to one aspect of the invention, there is provided a system for coating a flat substrate. One embodiment of this invention comprises: a coat head filled with liquid by capillary action, wherein the viscosity of the liquid is at least thirty centipoises; a pressurizer of the liquid that forms a meniscus at an orifice in the coat head; a contacter of the meniscus to the substrate; and a mover of the meniscus relative to the substrate.
According to a further embodiment of the invention, there is provided a system comprising: a coat head filled with liquid, wherein the viscosity of the liquid is at least thirty centipoises; a pressurizer of the liquid that forms a meniscus at an orifice of the coat head; a contacter of the meniscus to the substrate; a mover of the meniscus relative to the substrate; a regulator of liquid pressure at the base of the coat head; and a replenisher of liquid that replenishes liquid taken from the coat head with liquid in a reservoir.
According to another aspect of the present invention, there is provided a method for coating flat substrates. One embodiment of this aspect comprises: pressurizing liquid within a coat head wherein the liquid has a viscosity of at least thirty centipoises; forming a meniscus of liquid at an orifice in the coat head; contacting the meniscus of the liquid to the substrate; and moving the meniscus relative to the substrate.